MKAudioInput: set MKAudioDevice input to NULL before releasing it.
MKAudio: add MKAudioDelegate for controlling MKAudio state change decisions.
MKAudioInput: set MKAudioDevice input block after calling initializeMixer.

This is obviously racy. If our block is called before our call to
initializeMixer is done, it's accessing uninitialized memory.
MKAudioInput: fix mic boost memory corruption when resampling.
MKAudioInput: correctly preprocess resampled frames.
MKAudioInput: fix peakSignal miscalculation when resampling.
MKAudioInput: limit sidetone processing to 48KHz mic sampling rates.
src, doc: switch to Doxygen for MumbleKit documentation.
MKVoiceProcessingDevice: fix flakey VPIO on iOS 7 when built with the iOS 6.1 SDK.
MKAudio: add preferReceiverOverSpeaker setting.
MKAudio, MKAudioInput: add a setting to force CELT mode when using Opus.
MKCertificate: add +certificatesWithPKCS12:password: method; export raw data getters for certificate and private key.
MKAudio: add support for querying the audio mixer for debug info.
Merge remote-tracking branch 'upstream/master'